Methods to Enhance Separation Performance of Metal-Organic Framework Membranes

ABSTRACT

A method produces a metal-organic framework on a surface of another metal-organic framework. One embodiment comprises contacting the first metal-organic framework with a ligand and solvent solution; wherein the first metal-organic framework comprises a first ligand and a first metal; wherein the ligand and solvent solution comprises a second ligand that is different from the first ligand in the first metal-organic framework; and allowing the second ligand from the ligand and solvent solution to exchange with the first ligand present in the first metal-organic framework for a period of time suitable to produce the second metal-organic framework on the surface of the first metal-organic framework.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Application Ser. No. 61/947,923filed on Mar. 4, 2014, the disclosure of which is herein incorporated byreference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant numberCBET-1132157 awarded by the NSF. The government has certain rights inthe invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of gas separation and morespecifically to the construction of ultra-thin metal-organic frameworks(MOF), for example zeolitic-imidazolate frameworks (ZIF), for use as gasseparation membranes prepared by methods of counter diffusion andligand/metal exchange.

2. Background of the Invention

Metal-organic frameworks such as zeolitic-imidazolate frameworks, are aclass of organic-inorganic hybrid materials. The metal-organicframeworks are typically crystalline and have metal centers coordinatedto organic linkers. Metal-organic frameworks have been found useful forgas separation such as gas separation membrane applications.

Energy efficient membrane-based gas separations are attractivealternatives to conventional separation technologies such asdistillation. Despite the great potential of polycrystalline frameworkmembranes (such as MOF membranes) for energy efficient gas separations,the prohibitively high cost of these membranes and processes hamperedtheir practical applications. Amongst other reasons, the high cost maybe due to a lack of commercially-viable manufacturing processes and theunsatisfactory membrane performance (e.g., insufficient productivity andselectivity). For membrane applications, metal-organic frameworkmaterials are in the form of films on porous supports. Polycrystallinemetal-organic framework membranes are made by several different methods.As discussed above, such methods may have drawbacks for industrialapplications including the high cost of membranes and membraneproduction processes as compared to alternatives such as polymericmembranes.

Consequently, there is a need for improved synthesis methods for makingmembranes and films of metal-organic frameworks that address all of thedrawbacks described above.

BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS

These and other needs in the art are addressed in an embodimentcomprising a method for producing a second metal-organic framework on asurface of a first metal-organic framework. The method comprisescontacting the first metal-organic framework with a ligand and solventsolution; wherein the first metal-organic framework comprises a firstligand and a first metal; wherein the ligand and solvent solutioncomprises a second ligand that is different from the first ligand in thefirst metal-organic framework; and allowing the second ligand from theligand and solvent solution to exchange with the first ligand present inthe first metal-organic framework for a period of time suitable toproduce the second metal-organic framework on the surface of the firstmetal-organic framework.

These and other needs in the art are addressed in a further embodimentcomprising a method for producing a second metal-organic framework on asurface of a first metal-organic framework. The method comprisescontacting the first metal-organic framework with a metal and solventsolution; wherein the first metal-organic framework comprises a firstligand and a first metal; wherein the ligand and solvent solutioncomprises a second metal that is different from the first metal in thefirst metal-organic framework; and allowing the second metal from themetal and solvent solution to exchange with the first metal present inthe first metal-organic framework for a period of time suitable toproduce the second metal-organic framework on the surface of the firstmetal-organic framework.

These and other needs in the art are addressed in an additionalembodiment comprising a membrane. The membrane comprises a support, afirst metal-organic framework comprising a first ligand and a firstmetal, and a second metal-organic framework formed on the firstmetal-organic framework; wherein the second-metal organic frameworkcomprises a second ligand and a second metal.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter that form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand the specific embodiments disclosed may be readily utilized as abasis for modifying or designing other embodiments for carrying out thesame purposes of the present invention. It should also be realized bythose skilled in the art that such equivalent embodiments do not departfrom the spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

These drawings illustrate certain aspects of some of the embodiments ofthe present method, and should not be used to limit or define themethod.

FIG. 1 illustrates a counter-diffusion method in accordance with certainembodiments; and

FIG. 2 illustrates a ligand exchange reaction in accordance with certainembodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In embodiments, an ultra-thin “second” MOF membrane is created through acontrolled ligand and/or metal exchange process on the surface of a“first” MOF membrane attached to a support. As used herein, the term“first” refers to a prior existing MOF membrane, relative to the pointof formation of the “second” MOF membrane, which may be used in a ligandand/or metal exchange reaction to form the “second” MOF membrane on itssurface. As used herein, the term “second” refers to a MOF membranecreated on the surface of a prior existing “first” MOF membrane. Inembodiments, the first step of the technique is to create the first MOFmembrane on a support. The first MOF may be constructed on a supportusing a counter-diffusion method described below. Generally, theconstruction of the first MOF comprises contacting the support with afirst solution and a second solution. In embodiments, the support may besoaked with the first solution. The support may be contacted with thefirst solution by any suitable method. Without limitation, examples ofsuitable methods include spray, bath, submersion, slip, drop (i.e.,dropping the first solution on the support), spray coating, tapecasting, slip coating, and the like. The first solution may include anysolution suitable for foaming a desired metal-organic framework. Inembodiments, the first solution may include a metal and a solvent or aligand and a solvent. In embodiments, the support may then be contactedwith a second solution. The support may be contacted with the secondsolution by any suitable method. Without limitation, examples ofsuitable methods include spray, bath, submersion, slip, drop (i.e.,dropping the first solution on the support), spray coating, tapecasting, slip coating, and the like. The second solution may include anysolution suitable for forming a desired metal-organic framework. Inembodiments, the second solution may include a metal and a solvent or aligand and a solvent.

In embodiments, the order in which the solutions are applied isimmaterial. The metal and solvent solution may be applied first, or theligand and solvent solution may be applied first. In embodiments, theconstruction of the first MOF is dependent on the support beingcontacted by both the metal and the ligand. Therefore, it is to beunderstood that unless explicitly stated by an embodiment, the order ofsolutions applied to the support may be interchanged such that thesolution order may be reversed before contact with the support.

In embodiments, the first solution or the second solution may be a metaland solvent solution. The metal and solvent solution comprises a metal.Without limitation, examples of suitable metals may include, but are notlimited to iron, copper, zinc, cobalt, aluminum, zirconium, vanadium,chromium, manganese, the like, or any combinations thereof. The metalmay be any suitable metal for the desired metal-organic framework.Metals may be applied in combinations or may be present as a combinationin the metal and solvent solution. The metals may be provided by anysuitable metal source such as salts, (e.g.; nitrates, chlorides,acetates, etc.). For instance, an example of a suitable copper source iscopper nitrate hemi(pentahydrate), and an example of a suitable zincsource is zinc acetate dihydrate. In embodiments, the metals may bepresent in the metal and solvent solution in a range of about 0.01% toabout 5%; alternatively about 5% to about 10%; or alternatively about10% to about 20% by weight of the solution. With the benefit of thisdisclosure, one of ordinary skill in the art will be able to select ametal for a desired application.

In embodiments, the first solution or the second solution may be aligand and solvent solution. The ligand and solvent solution comprises aligand. Any suitable ligand may be used. Without limitation, examples ofsuitable ligands include imidazoles and derivatives such as2-methylimidazole, benzimidazole, nitroimidazole, chloroimidazole, andthe like; and carboxylic acids and derivatives such as1,4-benzenedicarboxylic acid, 1, 3, 5-benzene tricarboxylic acid,imidazole carboxaldehyde, the like, or any combinations thereof. Theligand may be any suitable ligand for the desired ligand-inorganicframework. Ligands may be applied in combinations or may be present as acombination in the ligand and solvent solution. As discussed below, theligand may also undergo post-synthetic modification. In embodiments theligands may be present in the ligand and solvent solution in a range ofabout 0.01% to about 20%; alternatively about 20% to about 40%; oralternatively about 40% to about 60% by weight of the solution. With thebenefit of this disclosure, one of ordinary skill in the art will beable to select a ligand for a desired application.

Any suitable solvent may be used for the metal and solvent solution andthe ligand and solvent solution. In embodiments, the solvents are anyorganic solvents suitable for metal-organic framework synthesis. Withoutlimitation, examples of suitable solvents are alcohols (i.e., methanol,ethanol, and the like), water, dimethylformamide, dimethyl sulfoxide, orany combinations thereof The choice of solvent is dependent upon thedesired application conditions. Solvents may be chosen to control thevapor pressure, evaporation rate, etc. Additionally, solvents may beused in combination to control the vapor pressure, evaporation rate,etc. The solvent may be present in the metal and solvent solution in arange of about 10% to about 50%; alternatively about 50% to about 70%;or alternatively about 70% to about 90% by weight of the solution. Thesolvent may be present in the ligand and solvent solution in a range ofabout 10% to about 50%; alternatively about 50% to about 70%; oralternatively about 70% to about 90% by weight of the solution. With thebenefit of this disclosure, one of ordinary skill in the art will beable to select a solvent for a desired application.

In some embodiments, a catalyst such as a deprotonator is also dissolvedin the metal and/or ligand solutions. Any catalyst suitable for themetal-organic framework may be used. Without limitation, examples ofcatalysts are deprotonators such as sodium formate; organic bases suchas ethylamine, diethylamine, and the like; inorganic bases such assodium hydroxide, potassium hydroxide, and the like; or any combinationsthereof. The catalyst may be added to the metal and solvent solutionand/or the ligand and solvent solution. Amongst other reasons, thecatalyst may be used to increase the reaction rate to insure that asufficient membrane develops prior to the reactants diffusing or thesolvent evaporating. The catalyst may be present in the metal andsolvent solution in a range of about 0% to about 10%; alternativelyabout 10% to about 20%; or alternatively about 20% to about 30% byweight of the solution. The catalyst may be present in the ligand andsolvent solution in a range of about 0% to about 10%; alternativelyabout 10% to about 20%; or alternatively about 20% to about 30% byweight of the solution. With the benefit of this disclosure, one ofordinary skill in the art will be able to select a catalyst for adesired application.

In embodiments, the first MOF comprises the metal and the ligand fromthe metal and solvent solution and the ligand and solvent solution. Inembodiments, the molar ratio of the metal:ligand:catalyst:solventsolution is about 1:X:Y:Z; where the ligand is represented by X and forevery 1 mole of metal is present in an amount of about 0.1 mole to about100 moles; where the catalyst is represented by Y and for every 1 moleof metal is present in an amount of about 0 moles to about 100 moles;and where the combined solvent amount (i.e. the total amount of solventof both the metal and solvent solution and the ligand and solventsolution) is represented by Z and for every 1 mole of metal is presentin an amount of about 10 moles to about 1000 moles. In some embodiments,the metal:catalyst:solvent solution is about 1:Y:Z; where the catalystis represented by Y and for every 1 mole of metal is present in anamount of about 0 moles to about 100 moles; and where the solvent isrepresented by Z and for every 1 mole of metal is present in an amountof about 10 moles to about 1000 moles. In an embodiment, theligand:catalyst:solvent solution is about 1:Y:Z; where the catalyst isrepresented by Y and for every 1 mole of ligand is present in an amountof about 0 moles to about 100 moles; and where the solvent isrepresented by Z and for every 1 mole of ligand is present in an amountof about 10 moles to about 1000 moles. With the benefit of thisdisclosure, one of ordinary skill in the art will be able to choose anappropriate molar ratio of components for a desired application.

In embodiments, the support may be any support (i.e., substrate) that issuitable for membrane-based separations. In an embodiment, the supportis porous for it to be suitable. In embodiments, the support maycomprise ceramics, polymers, stainless steel, and the like, or anycombinations thereof. The support may comprise any shape such as discs,hollow fibers, cylinders, sheets, tubes, tubules, tubulars, and thelike. It is to be understood that the shape and materials used for thesupport are not dependent upon one another, and a support of any shapemay comprise any material. With the benefit of this disclosure, one ofordinary skill in the art will be able to select a support for a desiredapplication.

In embodiments, the supports comprise pores. The pores may be coatedwith the first solution of either a ligand or metal before applicationof the second solution of either a ligand or metal. In embodiments, thepores in the support are sufficiently sized for the formation ofcrystals. In embodiments, the pores are from about 200 nm to about 1micron, alternatively from about 200 nm to about 500 nm, andalternatively from about 20 nm to about 200 nm. With the benefit of thisdisclosure, one of ordinary skill in the art will be able to choose asupport with a suitable pore size for a desired application.

Embodiments comprise a MOF. Examples of MOFs include, ZIF-7 (ZeoliticImidazolate Framework number 7), ZIF-8 (Zeolitic Imidazolate Frameworknumber 8), ZIF-9 (Zeolitic Imidazolate Framework number 9), ZIF-11(Zeolitic Imidazolate Framework number 11), ZIF-12 (Zeolitic ImidazolateFramework number 12), ZIF-21 (Zeolitic Imidazolate Framework number 21),ZIF-65 (Zeolitic Imidazolate Framework number 65), ZIF-67 (ZeoliticImidazolate Framework number 67), ZIF-71 (Zeolitic Imidazolate Frameworknumber 71), ZIF-76 (Zeolitic Imidazolate Framework number 76), ZIF-90(Zeolitic Imidazolate Framework number 90), ZIF-91 (Zeolitic ImidazolateFramework number 91), HKUST-1 (Hong Kong University of Science &Technology number 1), IRMOF-1 (Isoreticular Metal-Organic Frameworknumber 1), IRMOF-3 (Isoreticular Metal-Organic Framework number 3),MIL-101 (Materials for Institut Lavoisier number 101), UIO-66(University of Oslo number 66), and the like, or any combinationsthereof. Any MOF capable of being synthesized is suitable forembodiments. Also any MOF capable of being isostructrual with anotherMOF is also suitable for embodiments. With the benefit of thisdisclosure, one of ordinary skill in the art will be able to construct aMOF membrane for a desired application.

In embodiments, the first MOF formed may have a crystal size of about 10nm to about 10 pm. In an embodiment, the first MOF may have a thicknessof 100 nm to about 50 μm. With the benefit of this disclosure, one ofordinary skill in the art will be able to construct a first MOF membranewith the desired crystal size and thickness for a desired application.

Detailed techniques and examples of using the counter-diffusion methodare described below and in International Patent Application Serial No.PCT/US13/066221 filed on Oct. 22, 2013, the disclosure of which isincorporated herein in its entirety. As discussed above, thecounter-diffusion method comprises soaking the support in the metal andsolvent solution or the ligand and solvent solution. In embodiments, thesupport is soaked for a suitable time to fully saturate the pores insidethe support with the metal solution. As an example, a support may besoaked in a metal and solvent solution comprising ZnCl₂ dissolved inmethanol. In an embodiment, the rapid thermal deposition method(counter-diffusion method) includes solvothermally (or hydrothermally)treating the support saturated with the metal solution in acorresponding ligand and solvent or metal and solvent solution toprovide crystallization, thereby producing a metal-organic frameworkmembranes formed on the support. In the counter diffusion method, thesolvent does not evaporate (i.e. evaporation is impossible). Instead,the support is sealed in a reactor vessel (e.g. an acid-digestionvessel) under pressure. In the counter diffusion method, a catalyst maybe added (amongst other reasons) to increase the reaction rate so as toallow a sufficient membrane to be formed in the reaction zone, prior tothe ligand and/or metal diffusing away from the support. The temperaturemay range from about ambient temperature to about 200° C. It is to beunderstood that the first MOF membrane may be created in any fashion,and the embodiments disclosed herein allow for the creation of a secondMOF membrane on the surface of a first MOF membrane that was created inany fashion. As such, the second MOF membrane and method of producingthe second MOF membrane is not limited or restricted in any manner bythe method of producing the first MOF membrane.

FIG. 1 illustrates a counter-diffusion technique. In FIG. 1, a support 5that is metal saturated is lowered into a ligand and solvent solution10. The contra-diffusion reaction starts in the reaction zone 35 of thesupport 5 that is metal saturated, as the metal diffuses out of theinterior of the support 5 and the ligand from the ligand and solventsolution 10 diffuses into the support 5. A first MOF membrane 30 formsin the reaction zone 35 that comprises the ligand and the metal from thesupport 5.

Once a first MOF membrane is created, an ultra-thin second MOF membranemay be constructed at the outermost surface of the first MOF membrane.This process utilizes ligand and/or metal exchange reactions. Inembodiments, the ligand and/or metal exchange reaction is initiated byplacing the first MOF membrane into a ligand and solvent solution, ametal and solvent solution, or a ligand and metal solvent solution,depending on which components: ligand and/or metal are desired forexchange.

In embodiments, any of the MOFs described above that are suitable forthe first MOF are also suitable for the second MOF. In embodiments, thesecond MOF is isostructural with the first MOF.

The description of the ligand and solvent solutions and the metal andsolvent solutions used to construct the second MOF membrane is exactlythe same (e.g. same grouping of solvents, ligands, metals, catalysts,ratio of components, etc.) as the ligand and solvent solutions and themetal and solvent solutions described above for the construction of thefirst MOF membrane. However, in embodiments, the solutions comprisedifferent ligands for a ligand exchange reaction, different metals for ametal exchange reaction, and different ligands and metals for a ligandand metal exchange reaction than the ligands and/or metals used toconstruct the first MOF membrane. For example, in embodiments comprisinga ligand exchange reaction, if a ligand and solvent solution comprisingthe ligand imidazole-2-carboxaldehyde was used to create the first MOFmembrane, the ligand and solvent solution used to create the second MOFmembrane would not comprise the ligand imidazole-2-carboxaldehyde, butinstead would comprise a different ligand, e.g., 2-methylimidazole. Asanother example, in embodiments comprising a metal exchange reactionwherein it is desired to construct a second MOF comprising a differentmetal than that of the first MOF, if a metal and solvent solutioncomprising the metal cobalt was used to create the first MOF membrane,the metal and solvent solution used to create the second MOF membranewould not comprise the metal cobalt, but instead would comprise adifferent metal, e.g., zinc. Finally, and unlike the solutions used forthe creation of the first MOF membrane, a ligand and metal solventsolution may be used to exchange both the ligand and metal of the firstMOF in the creation of the second MOF. Such a solution will be referredto as a “precursor solution” because said solution contains bothprecursor materials used to create the second MOF. In embodimentscomprising both a ligand and metal exchange reaction, the surface of thefirst MOF membrane will exchange both ligand and metal groups to createan ultra-thin second MOF membrane on its surface that comprisesdifferent ligands and metals than that of the first MOF membrane. Inembodiments, when constructing the second MOF membrane, the ligand andsolvent solution, the metal and solvent solution, or the precursorsolution may comprise a single species of ligand, metal, or ligand andmetal respectively. Alternatively, when constructing the second MOFmembrane, the ligand and solvent solution, the metal and solventsolution, or the precursor solution may comprise multiple species ofligands, metals, or ligands and metals respectively, as such, themultiple species of ligands and/or metals may react simultaneously toconstruct the second MOF membrane as desired.

The exchange reactions described above create an ultra-thin second MOFon the surface of the first MOF by exchanging the ligands and/or metalsof the surface unit cells of the first MOF. Without being limited bytheory, this process does not induce the formation of new crystals, butinstead merely replaces the ligands and/or metals of the uppermostportion of the unit cells of the first MOF to create a new ultra-thinMOF. These exchange reactions therefore allow for the creation of a newMOF that is isostructural to the first yet comprises an altered poresize. The altered pore size of the second MOF may be larger or smallerthan the pore size of the first MOF. Generally, larger pore sizes wouldbe expected to increase permeability, conversely smaller pore sizes mayincrease selectivity. Without limitation, creating MOF membranes withthe right mix of permeability and selectivity is important forapplications such as gas selectivity.

FIG. 2 illustrates an example of the ligand exchange reaction describedabove. First MOF membrane 30 has been formed via a counter diffusionmethod on support 5 (see FIG. 1 for a description). Imidazole 40 of anisostructural ZIF-8 membrane is exchanged for 2-methylimidazolate 45 viaa controlled ligand exchange reaction 50 to form a second MOF membrane55. Since the ligand exchange reaction 50 is tightly controlled, thewidth of the second MF membrane is significantly thinner than that ofthe first MOF membrane. Furthermore, the ligand exchange reactionproduces a second MOF membrane with a pore size that is different fromthat of the first MOF membrane. This alteration of pore size may createa second MOF membrane with either increased permeability or increasedselectivity, dependent upon whether the second MOF membrane pore sizehas decreased or increased relative to the first MOF membrane pore size.For example, a first ZIF-8 membrane comprising a pore size greater than4.1 angstroms may undergo ligand exchange to produce a second ZIF-8membrane comprising a pore size between 4.0 and 4.1 angstroms.Decreasing the pore size decreases permeability but increasesselectivity. For example, in hydrocarbon gas separation applications, apore size greater than 4.1 angstroms may not comprise any selectivitybetween propane and propylene; however, a pore size between 4.0 and 4.1angstroms may possess selectivity between propane and propylene.Therefore, in the example described above, the first ZIF-8 membrane maybe more permeable to both propane and propylene but not selectivetowards either, whereas the second ZIF-8 membrane may be overall lesspermeable to propane and therefore possess a selectivity towardspropane/propylene that is not present in the first ZIF-8 membrane. Theabove process and result may be similar for metal exchange andligand/metal exchange reactions.

Although the methods described herein are couched in terms of a bilayerof a first MOF and a second MOF, those of ordinary skill in the art willrecognize and appreciate that the methods described herein may also beused to create a third MOF on the surface of the second MOF, a fourthMOF on the surface of the third MOF, and so forth. Thus, the methodsdescribed herein may also be used to create trilayers, quadlayers, etc.of isostructural membranes, with each subsequent layer possessingdifferent ligands and/or metals than the layer on which it isconstructed. As such, the methods described herein may be used to createmulti-layered MOF membranes with varying pore sizes, thicknesses, andcompositions, which may be used in various applications as describedherein.

In embodiments, the first MOF membrane may possess a larger pore sizethan the second MOF membrane. Alternatively, the first MOF membrane maypossess a smaller pore size than the second membrane. Therefore, therelationship between the pore sizes of first and second MOFs isirrelevant, and the second MOF membrane may possess a pore size largeror smaller than the corresponding first MOF membrane. The pore sizes aredependent entirely upon which metal and ligands were used to produce thefirst MOF and the second MOF. In embodiments, factors for considerationin choosing which ligands/metals to use for the second MOF membraneinclude whether the ligand/metal exchange reaction will produce a secondMOF membrane with the desired pore size relative to the first MOFmembrane, and whether the second MOF membrane will not alter the crystalstructure of the underlying first MOF membrane but remain isostructuralto it.

In embodiments, the second MOF membrane may possess a thickness of lessthan about 500 nm. Therefore, the thickness of the second MOF membranemay be about 500 nm, about 250 nm, about 100 nm, about 50 nm, about 10nm, or less. With the benefit of this disclosure, one of ordinary skillin the art will be able to prepare a second MOF membrane having asuitable thickness for an application.

In embodiments, the ligand and/or metal exchange reaction is controlledsuch that the rate of reaction proceeds only as allowed. This isaccomplished by making the ligand and/or metal exchange reaction adiffusion limited process. For example, given a fixed time for thereaction, the concentration of the ligand and/or metals in the ligandand solvent, metal and solvent, or precursor solutions may be decreasedsuch that only the uppermost surface of the unit cells of the first MOFmembrane are exchanged. Alternatively, given a fixed concentration ofreactants, the time may be limited so that there is only enough time forthe uppermost surface of the unit cells of the first MOF membrane to beexchanged. Likewise, temperature may be controlled. Temperature and thereaction rate exist in a direct relationship, such that lowering thetemperature may lower the reaction rate and thus potentially limit theligand/metal exchange reaction to only the uppermost surface of the unitcells of the first MOF membrane. The specific reaction conditions for aligand and/or metal exchange reaction vary with the ligand(s) and/ormetal(s) chosen as reactants. For example, when a first membrane isZIF-8 and an exchanging ligand is imidazole-2-carboxaldehyde, thetemperature and time to reach 25% ligand exchange of a first membranemay be around 60° C. and 48 hours. The reaction process may be tailoredto provide higher exchange rates or lower exchange rates as desired.With the benefit of this disclosure, one of ordinary skill in the artwill be readily able to determine the proper reaction conditions for aspecific ligand and/or metal exchange reaction.

In embodiments, since the disclosed methods conserve metal and/or ligandreagents and use less metal and ligand reagents than currently knownmethods; the metal and solvent solution and/or the ligand and solventsolution may be recycled as the solutions may comprise sufficientreagent to maintain sufficient reactivity for additional uses. Inembodiments, this recycling may comprise reuse of the metal and solventsolution and/or the ligand and solvent solution, or it may comprisecombining the used metal and solvent solution and/or the ligand andsolvent solution with another metal and solvent solution and/or ligandand solvent solution.

In embodiments, the ligands of the second MOF membrane may undergopost-synthetic modification. For example, 2-methyl imidazolate of thefirst MOF membrane may be exchanged with imidazole-2-carboxaldehyde. Thealdehyde functional group of imidazole-2-carboxaldehyde may then reactwith an amine functional group by condensation reaction. The aminefunctional group may further comprise polar functional groups such ashydroxyl and carboxyl groups, creating a polar membrane surface that maybe used to increase selectivity, e.g., propylene with weak polaritymight be favored over non-polar propane.

In some embodiments, and as discussed above, the first MOF and thesecond MOF may form a selectively permeable membrane which may allow forthe isolation of a specific gas. Thus, the above described method allowsfor the separation of and consequently, the selection of gases. Gaseswhich may be selected by this method include, but are not limited tosuch as H₂, CO₂, N₂, CH₄, SF₆, C₃H₆, C₃H₈, C₂H₄, C₂H₆, n-C₄H₁₀, i-C₄H₁₀,and the like. The gases may be selected for and isolated from mixturesof any gas (e.g., selecting C₂H₄ from a mixture of C₂H₄ and C₂H₆, orselecting CO₂ from air). As described above, the mixtures of any gas mayinclude species of gases disclosed herein, or may include mixtures ofspecies of gases disclosed herein, or may include any such gases ormixtures of gases whether or not said gases or mixtures of gases aredisclosed herein.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations may be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

It should be understood that the compositions and methods are describedin terms of “comprising,” “containing,” or “including” variouscomponents or steps, the compositions and methods can also “consistessentially of” or “consist of” the various components and steps.Moreover, the indefinite articles “a” or “an,” as used in the claims,are defined herein to mean one or more than one of the element that itintroduces.

For the sake of brevity, only certain ranges are explicitly disclosedherein. However, ranges from any lower limit may be combined with anyupper limit to recite a range not explicitly recited, as well as, rangesfrom any lower limit may be combined with any other lower limit torecite a range not explicitly recited, in the same way, ranges from anyupper limit may be combined with any other upper limit to recite a rangenot explicitly recited. Additionally, whenever a numerical range with alower limit and an upper limit is disclosed, any number and any includedrange falling within the range are specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues even if not explicitly recited. Thus, every point or individualvalue may serve as its own lower or upper limit combined with any otherpoint or individual value or any other lower or upper limit, to recite arange not explicitly recited.

Therefore, the present embodiments are well adapted to attain the endsand advantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, and may bemodified and practiced in different but equivalent manners apparent tothose skilled in the art having the benefit of the teachings herein.Although individual embodiments are discussed, the invention covers allcombinations of all those embodiments. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. Also, the terms in the claimshave their plain, ordinary meaning unless otherwise explicitly andclearly defined by the patentee. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of the invention. If there is any conflict in the usages of aword or term in this specification and one or more patent(s) or otherdocuments that may be incorporated herein by reference, the definitionsthat are consistent with this specification should be adopted.

What is claimed is:
 1. A method for producing a second metal-organicframework on a surface of a first metal-organic framework comprising:(A) contacting the first metal-organic framework with a ligand andsolvent solution; wherein the first metal-organic framework comprises afirst ligand and a first metal; wherein the ligand and solvent solutioncomprises a second ligand that is different from the first ligand in thefirst metal-organic framework; and (B) allowing the second ligand fromthe ligand and solvent solution to exchange with the first ligandpresent in the first metal-organic framework for a period of timesuitable to produce the second metal-organic framework on the surface ofthe first metal-organic framework.
 2. The method of claim 1, wherein thesecond ligand in the ligand and solvent solution comprises an imidazole,a carboxylic acid, or any combinations thereof.
 3. The method of claim1, wherein the ligand and solvent solution further comprises a catalyst,wherein the catalyst comprises an amine, an organic base, an inorganicbase, or any combinations thereof.
 4. The method of claim 1, wherein thesolvent comprises water, an alcohol, dimethylformamide, dimethylsulfoxide, or any combinations thereof.
 5. The method of claim 1,wherein a pore size of the second metal-organic framework is differentfrom a pore size of the first metal-organic framework.
 6. The method ofclaim 1, further comprising isolating a gas from a mixture of gases,wherein the gas comprises H₂, CO₂, N₂, CH₄, SF₆, C₃H₆, C₃H₈, C₂H₄, C₂H₆,n-C₄H₁₀, i-C₄H₁₀, or any combinations thereof wherein the isolating thegas is accomplished by a selectively permeable membrane formed from thefirst metal-organic framework and the second metal-organic framework. 7.The method of claim 1, wherein the ligand and solvent solution furthercomprises a second metal, wherein the second metal comprises iron,copper, zinc, cobalt, aluminum, zirconium, vanadium, chromium,manganese, or any combinations thereof.
 8. A method for producing asecond metal-organic framework on a surface of a first metal-organicframework comprising: (A) contacting the first metal-organic frameworkwith a metal and solvent solution; wherein the first metal-organicframework comprises a first ligand and a first metal; wherein the ligandand solvent solution comprises a second metal that is different from thefirst metal in the first metal-organic framework; and (B) allowing thesecond metal from the metal and solvent solution to exchange with thefirst metal present in the first metal-organic framework for a period oftime suitable to produce the second metal-organic framework on thesurface of the first metal-organic framework.
 9. The method of claim 8,wherein the second metal in the metal and solvent solution comprisesiron, copper, zinc, cobalt, aluminum, zirconium, vanadium, chromium,manganese, or any combinations thereof.
 10. The method of claim 8,wherein the metal and solvent solution further comprises a catalyst,wherein the catalyst comprises an amine, an organic base, an inorganicbase, or combinations thereof.
 11. The method of claim 8, wherein thesolvent comprises water, an alcohol, dimethylformamide, dimethylsulfoxide, or any combinations thereof.
 12. The method of claim 8,wherein a pore size of the second metal-organic framework is differentfrom a pore size of the first metal-organic framework.
 13. The method ofclaim 8, further comprising isolating a gas selected from a mixture ofgases, wherein the gas comprises H₂, CO₂, N₂, CH₄, SF₆, C₃H₆, C₃H₈,C₂H₄, C₂H₆, n-C₄H₁₀, i-C₄H₁₀, or combinations thereof; wherein theisolating the gas is accomplished with the first metal-organic frameworkand the second-metal organic framework.
 14. The method of claim 8,wherein the metal and solvent solution further comprises a secondligand, wherein the second ligand comprises an imidazole, a carboxylicacid, or any combinations thereof.
 15. A membrane comprising: a support,a first metal-organic framework comprising a first ligand and a firstmetal, and a second metal-organic framework formed on the firstmetal-organic framework; wherein the second-metal organic frameworkcomprises a second ligand and a second metal.
 16. The membrane of claim15, wherein the second ligand is a different ligand from the firstligand.
 17. The membrane of claim 15, wherein the second metal is adifferent metal from the first metal.
 18. The membrane of claim 15,wherein a pore size of the first metal-organic framework is differentfrom a pore size of the second metal-organic framework.
 19. The membraneof claim 15, wherein the second ligand comprises an imidazole, acarboxylate, or any combinations thereof.
 20. The membrane of claim 15,wherein the second metal comprises iron, copper, zinc, cobalt, aluminum,zirconium, vanadium, chromium, manganese, or any combinations thereof.